In a system including an image signal transmitter and an image display unit for displaying images based on an image signal transmitted by the image signal transmitter, the image signal is often transmitted through a cable connecting the image signal transmitter and the image display unit. When an analog image signal is transmitted through the cable, the image signal degrades depending on the cable characteristics or a cable length, and correction becomes necessary.
Frequency degradation which is caused by transmission through a cable is corrected by transmitting an image signal to provide a reference, comparing it with an image signal that is included inside of a receiver, as a reference to acquire frequency degradation that is produced in a transmission path, and emphasizing a high-frequency component. There is such an image display unit disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 08-331419).
The image display unit disclosed in Patent Document 1 uses, as a reference signal for waveform equalization, an impulse signal including a roll-off rate of 10% and that is superimposed during a vertical retrace line interval of an image signal, carries out a correlation computation using an ideal impulse signal in a time axis to successively correct a tap coefficient of a transversal filter, and thereby executes waveform equalization, and also measures a signal-to-noise (S/N) ratio of a signal to vary a high-frequency characteristic of the reference signal for waveform equalization corresponding to the S/N ratio.
However, the technique disclosed in Patent Document 1 is a technique used for broadcast radio, and because a band of an image signal transmitted by a transmitting end is limited, the technique cannot be directly applied to a system whose band is not limited such as, for example, an image signal output of a personal computer (PC).
The output image signal of a PC includes a signal including a different number of pixels for screen display such as Video Graphic Array (VGA), Super Video Graphic Array (SVGA), Extended Video Graphic Array (XGA) and Super Extended Video Graphic Array (SXGA), and also a signal including a different number of scanning frequencies (vertical frequency, horizontal frequency, dot clock) even when the signal has the same number of pixels for the screen display. As a technique that is intended to correct an image signal supplied from a PC, there is a technique disclosed in Patent Document 2 (Japanese Patent Laid-Open No. 2000-305506).
A display unit disclosed in Patent Document 2 calculates the pixel frequency of an image signal supplied from a PC, measures a characteristic of a transmission filter system by sampling at N points per pixel using a dot clock including a frequency N times as many as the pixel frequency, and passing through a digital filter including a filter coefficient defining an inverse characteristic to the transmission filter system set thereto, thereby reproducing the image signal.    [Patent Document 1] Japanese Patent Laid-Open No. 08-331419    [Patent Document 2] Japanese Patent Laid-Open No. 2000-305506